The goal of the proposed research is to purify and study the properties of bovine enterokinase. The properties of the bovine enzyme will be compared with those of the porcine enzyme, described recently by Desnuelle and his group, and with those of trypsin, thrombin, and other serine proteinases. We propose to determine the partial amino acid sequence of specific regions of the active site serine and histidine and the residues surrounding the half-cystine residues. Limited cleavage of disulfide bonds will be tried to attempt a separation of the two subunits. We will attempt to isolate the catalytic unit under conditions which should maintain the native structure. Studies on the isolated chain will attempt to determine if it is enzymatically active, and, if so, if it will be modified by the heavy chain. The properties of the catalytic subunit will be compared to those of other serine proteinases.